Such drones have multiple rotors driven by respective motors that can be controlled independently in order to control the attitude and the speed of the drone.
A typical example of such a drone is the AR.Drone from Parrot SA, Paris, France, which is a quadricopter fitted with a series of sensors (accelerometers, three-axis gyros, altimeter). The drone also has a forward-looking camera picking up an image of the scene towards which the drone is heading, and a vertically-pointed camera picking up an image of the overflown terrain.
The drone is piloted by the user by means of a remote-control device—referred to below as an “appliance”—that is connected to the drone by a radio link.
WO 2010/061099 A1 (Parrot SA) in particular describes such a drone and how it can be piloted via a media player or a telephone having a touch screen and an incorporated accelerometer, e.g. a cell phone of the iPhone type or a media player or a multimedia tablet of the iPod Touch or iPad type (trademarks registered by Apple Inc., USA). Those appliances incorporate the various control members needed for detecting piloting commands and for bidirectional exchange of data with the drone via a local network wireless link of the type provided by WiFi (IEEE 802.11) or Bluetooth (registered trademarks). In particular, the appliance is provided with a touch screen that displays the image picked up by the forward-looking camera, and having superposed thereon a certain number of symbols that enable commands to be activated by mere contact of the user's finger on the touch screen. The display also enables “immersive piloting” in which the user, instead of piloting the drone while looking at the drone, makes use of the image from the camera as though the pilot were on board the drone.
The invention relates more particularly to automatically executing flip maneuvers of the “roll” type (rotation of the drone through a complete revolution about its roll axis), or of the “loop” type (rotation of the drone through a complete revolution about its pitching axis).
The roll may be to left or to right depending on the direction of rotation. It may also be constituted by a sequence of complete revolutions one after another; the description below relates to a maneuver constituted by a single revolution, however that description is not of a limiting character. The description also relates to the maneuver being performed from a configuration in which the drone is initially stationary, hovering, but that configuration is not limiting either: the roll may be performed for example together with a horizontal speed component, with a point on the drone then describing a trajectory in an absolute frame of reference that is helical rather than circular.
The loop may be forwards or backwards, depending on whether the rotation is initiated by the drone performing a nose-up or a nose-down maneuver, respectively. It should be observed that a loop differs from a roll only by the axis of rotation involved (pitching axis instead of roll axis). As a result, the description below relates only to executing a roll, but everything that is mentioned concerning a roll can be transposed to executing a loop, mutatis mutandis, with a different axis of rotation being selected.
The article by S. Lupashin et al. “A simple learning strategy for high-speed quadrocopter multi-flips”, Proceedings of the 2010 IEEE International Conference on Robotics and Automation, May 2010, pp. 1642-1648 describes how to control a drone of the quadricopter type in order to perform such a maneuver.
The technique described in that document consists in imparting an initial rotary impulse to the drone, with the magnitude of the impulse being calculated so that the drone reaches a final altitude that is as close as possible to the horizontal at the end of the maneuver.
Nevertheless, since the roll is executed by the drone in an open loop (no servo-control), it is not guaranteed in any way that the drone will indeed be horizontal at the end of the maneuver, or in other words that its angular speed will be zero after pivoting through 360°.
In order to mitigate that risk of overshoot (more than one complete revolution) or of undershoot (less than one complete revolution), the authors propose using successive iterations to adjust the durations of the various sequences for controlling the roll so as to come as close as possible to the ideal figure: one complete revolution, neither more nor less, with zero final angular velocity after rotating through 360°.
As a result, it is not possible to achieve the looked-for result on the first attempt. On the contrary, that requires a large number of successive approximations, and according to that article about 40 to 50 iterations of the adjustment algorithm.
Furthermore, even after adjusting the various parameters, proper execution of the maneuver can be disturbed by various external factors such as gusts of wind, turbulence close to a wall, etc.
The article by J. H. Gillula et al. “Design of guaranteed safe maneuvers using reachable sets: autonomous quadrotor aerobatics in theory and practice”, Proceedings of the 2010 IEEE International Conference on Robotics and Automation, May 2010, pp. 1649-1654, describes a comparable technique for causing a quadricopter to execute back-flips during horizontal flight, however that technique presents the same drawbacks and limitations as those explained above.
In practice, proper execution of a roll or a loop encounters several difficulties.
One of them is associated with the fact that when the motors are controlled to cause the drone to perform the desired rotation, as a result of the reversal of left/right or forward/backward thrust that serves to start the rotation about the roll axis or the pitching axis, the drone is no longer supported and will therefore lose altitude between the beginning and the end of the maneuver (unlike performing one complete revolution about a yaw axis, where the drone remains substantially flat throughout the rotation).
As described in the above-mentioned article by Lupashin, that difficulty can be resolved by controlling the motors of the drone simultaneously in such a manner as to impart a prior upward vertical thrust impulse thereto before beginning the rotation. This gives sufficient vertical impetus to the drone to ensure that once it has finished off the maneuver, it is substantially back at the same altitude as it had initially.
However this prior vertical thrust that is applied in an open loop to all four motors does not avoid the risk of being offset sideways in the event of the attitude of the drone not being strictly horizontal at the moment the impulse is applied.
As explained above, another difficulty lies in accurate looping rotation through one complete revolution, i.e. rotation through 360° without angular overshoot, which would then give rise to the drone oscillating about its final horizontal position. That drawback is particularly marked when the maneuver is performed quickly—but such speed is essential not only for accentuating the spectacular nature of the maneuver, but also, and above all, for limiting the above-mentioned effects of losing support during the execution of the maneuver.